1. Field of the Invention
This invention pertains to dielectric logging measures made on a subterranean formation and, more specifically, to a method for calibrating dielectric permittivity data taken with logging tools in a frequency range of from about 10 to about 100 MHz.
2. Technical Background
Current logging tools that measure the dielectric permittivity of reservoir formations normally operate over a wide range in signal frequency from 16 MHz to 1100 MHz. The permittivity measurements from these tools are used to determine the water and oil saturations of reservoir formations. However, the dielectric permittivity of a formation changes with the signal frequency so that an equation that correctly predicts the water and, hence, oil saturation from the formation dielectric permittivity at one frequency may predict incorrect values at other frequencies.
One method, based on the Hanai-Bruggeman equation, has been developed which uses the same algebraic expression to relate the dielectric permittivity to water saturation. The method is set forth in my earlier patent application Ser. No. 871,730, filed June 6, 1986, now U.S. Pat. No. 4,786,873 and entitled "Improved Method for Evaluating Water Saturation in Petroleum Reservoirs from Dielectric Permittivity." The disclosure in Ser. No. 871,730, now U.S. Pat. No. 4,786,873 is incorporated herein by reference. The method has adjustable parameters to compensate for the frequency dependence of the permittivity. The adjustable parameters are determined from laboratory measurements taken from core samples. However, laboratory measurements of the dielectric permittivity below 100 MHz are often inaccurate because of limitations in laboratory techniques.